1.2 Rainwater harvesting in situ

In arid and semi-arid regions, where precipitation is low or
infrequent during the dry season, it is necessary to store the maximum amount of
rainwater during the wet season for use at a later time, especially for
agricultural and domestic water supply. One of the methods frequently used in
rainwater harvesting is the storage of rainwater in situ. Topographically
low areas are ideal sites for in situ harvesting of rainfall. This
technique has been used in the arid and semi-arid regions of northeastern
Brazil, Argentina, and Paraguay, primarily for irrigation purposes. The in
situ technology consists of making storage available in areas where the
water is going to be utilized.

Technical Description

All rainfall harvesting systems have three components: a
collection area, a conveyance system, and a storage area. In this application,
collection and storage is provided within the landscape. Topographic depressions
represent ideal collection and storage areas. In many situations, such areas are
impermeable, being underlain by clay soils that minimize infiltration. Methods
of rainwater harvesting in situ, including site preparation of
agricultural areas in Brazil, are described below.

· Use of Topographic Depressions as Rainfall Harvesting
Areas

In Paraguay, areas of low topography used for rainwater storage
are known as tajamares. Tajamares are constructed in areas with clay
soils at least 3 m deep. The tajamares are served by distribution canals
that convey water from the storage area to the areas of use. The collection and
storage areas need to be fenced to avoid contamination by animals. This
technology is usually combined with storage tanks built of clay. The water is
delivered from the in situ rainfall collection area to the storage tank
by means of a pump, usually driven by a windmill, as shown in Figure 3.

Figure 3: Low Topography Rainfall
Harvesting Areas (Tajamar).

Source: Eugenio Godoy V., National Commission on
Integrated Regional Development of the Paraguayan Chaco, Filadelfia,
Paraguay.

· Use of Furrows as Rainwater Storage Areas

Furrows may be used as an in situ means of storing
harvested rainwater. They are built prior to or after planting to store water
for future use by the plants. A variation on the use of topographic depressions
to store rainfall, this method uses flattened trenches between the rows of crops
to store water (Figures 4a-4c). Furrows may have mud dams or barriers every 2 m
to 3 m along the row in order to retain water for longer periods of time and
avoid excessive surface runoff and erosion (Figure 4d). Raised beds may also be
used to trap the water in the furrows, or uncultivated areas may be left between
rows, spaced at 1 m apart, to assist in capturing rainwater falling on the land
surface between furrows (Figures 4e and 4f).

Figure 4: Site Preparation Methods for in situ Rainwater
Harvesting in Northeastern Brazil.

The Guimares Duque method was developed in Brazil during
the 1950s, and uses furrows and raised planting beds, on which cross cuts to
retain water are made using a reversible disk plow with at least three disks.
The furrows are usually placed at the edge of the cultivation zone (Figure 5).

Figure 5: Guimares Duque Site
Preparation Method.

Source: Josos Anjos Barbosa, EMBRAPA, Petrolina,
PE, Brazil, 1995.

Extent of Use

This technology has been extensively used in northeastern Brazil,
in the Chaco region of Paraguay, and in Argentina. It can be used to augment the
water supply for crops, livestock, and domestic use. With the mechanization of
agriculture, its use has diminished, but it is still recommended for regions
where the volume of rainfall is small and variable. The approach used depends
primarily on the availability of equipment, the nature of the agricultural and
livestock practices, and the type of soil.

Water stored in tajamares is normally used for livestock
watering and may be used for domestic consumption after filtration and/or
chlorination. Individual tajamares have also been used as a means of
artificially recharging groundwater aquifers. Tajamares built in the
Paraguayan Chaco have produced up to 6 800 m3/yr for aquifer
recharge.

Operation and Maintenance

This technology requires very little maintenance once the site is
chosen and prepared. Maintenance is done primarily during the course of normal,
day-to-day agricultural activities, and consists primarily of keeping the
collection area free of debris and unwanted vegetation. Where only parts of the
rows are cultivated, rotating the areas that are plowed will enable more
efficient maintenance of the available storage area.

Level of Involvement

This technology is simple and easy to use. Governmental
organizations and the agricultural community generally work together to support
and promote the in situ rainwater storage. Educational and information
programs should be provided to inform users of the benefits of this technology,
and the means of implementing rainwater harvesting while preventing soil loss.

Costs

The costs of in situ rainwater collection systems are
minimal. The main cost of this technology is in the equipment and labor required
to build the fences and furrows. Table 1 shows representative costs reported for
different methods of site preparation in cultivated areas of Brazil. Further,
the construction cost of a tajamar in Paraguay has been reported at $4
500. This cost includes not only the cost of soil preparation, but also the cost
of ancillary equipment such as the storage tank and windmill shown in Figure 3.

Table 1 - Estimated Cost ($) of Different Site Preparation
Methods for Rainwater Collection Areas in Agricultural Areas of Brazil

Method

Basic Equipment

Animal Traction

Total

Hourly Cost of Implementation

Flat terrain trenches

150.00

300.00

450.00

0.96

Post-planting furrows

80.00

300.00

380.00

0.90

Pre-planting furrows

180.00

70.00

250.00

0.90

Furrows with barriers

180.00

70.00

250.00

0.90

Inclined raised beds

1 500.00

1000.00

2 500.00

12-15

Furrows in partial areas

100.00

80.00

180.00

0.70

Guimar Duque method

-

-

-

12-15

Effectiveness of the Technology

This technology increases water supply for irrigation purposes in
arid and semi-arid regions. It promotes improved management practices in the
cultivation of corn, cotton, sorghum, and many other crops. It also provides
additional water supply for livestock watering and domestic consumption.

Suitability

This technology is applicable to low topographic areas in arid or
semi-arid climates.

Advantages

· This technology requires minimal additional
labor.

· It offers flexibility of implementation; furrows can be
constructed before or after planting.

· Rainwater harvesting is compatible with agricultural best
management practices, including crop rotation.

· It provides additional flexibility in soil utilization.

· Permeable in situ rainwater harvesting areas can be
used as a method of artificially recharging groundwater aquifers.

Disadvantages

·In situ rainwater harvesting cannot be
implemented where the slope of the land is greater than 5%.

· It is difficult to implement in rocky soils.

· Areas covered with stones and/or trees need to be cleared
before implementation.

· The additional costs incurred in implementing this
technology could be a factor for some farmers.

· It requires impermeable soils and low topographic relief
in order to be effective.

· The effectiveness of the storage area can be limited by
evaporation that tends to occur between rains.

Cultural Acceptability

In situ rainfall harvesting has been practiced for many
years by the agricultural communities of northeastern Brazil, Paraguay, and
Argentina. Agricultural communities in other arid and semi-arid regions can
readily improve their level of irrigation and increase their production yield
using this technique.

Further Development of the Technology

The equipment used in the construction of the furrows and storage
areas must be improved. Relatively inexpensive plows and tractors can reduce the
cost of implementation and contribute to the more widespread use of this
technology by small farmers. New methods of soil conservation should be
explored.